NEW YORK’S “20-FOOT CLAY” CONUNDRUM: WHICH IS WRONG – PUBLISHED DATES, PRESUMED GLACIAL EVENTS, OR STRATA ORIGINS?

Fuller (1914) mapped the surface geology of Long Island and interpreted the regional glacial events. This mapping has been reinterpreted many times since, each generally fitting in with the main known regional strata (Smolensky & Others 1989). However, when the presence of the 20-Foot Clay is considered, for most of these versions either the presumed ages or origin of the strata don’t add up.

The 20-Foot Clay, as it is commonly known, is a marine clay mapped along the south shore of Long Island (Doriski & Wilde-Katz 1983). Mentioned in references, but not mapped (Buxton & Shernoff 1999), data in MRCE archives confirms it is a discontinuous unit below Holocene deposits that extends into Queens and Brooklyn. Rampino & Sanders (1981) studied it and adjacent strata in more detail, renaming the clay the Wantagh Fm.

The following Pleistocene strata are mapped in the region: Jameco outwash is in western Long Island, and published references generally consider it to be Illinoian age, along with the Montauk till in the vicinity of, and presumably below, the Ronkonkoma moraine in eastern LI. Gardiners Clay, found south of the Ronkonkoma and above the Jameco, is an interglacial marine clay generally considered to be Sangamon in age, though there are significant conflicts in the published dating (AAR dates are MIS stages 9 to 5, ¹⁴C dates are MIS 3 or older). A sand layer identified as outwash (Rampino’s Merrick Fm.) lies above the Gardiners. The marine 20-Foot Clay/Wantagh Fm. lies above this sand (AAR date MIS 5 or possibly 3, ¹⁴C date ~28,150 BP). A layer of outwash (Rampino’s Bellmore Fm.) lies above the Wantagh, and can be traced to the LGM outwash on the surface of Long Island.

INTERPRETATIONS: If there was only one Wisconsinan glaciation in the NYC region, then Wantagh is a Sangamon interglacial deposit, Merrick is the Illinoian outwash, Gardiners is pre-Illinoian interglacial, and Jameco is from a pre-Illinoian glaciation. If Gardiners is Sangamon, then there are strata (Merrick and Wantagh) present from an unidentified extra glaciation and interstadial (MIS 4 & 3?) prior to the LGM advance. The only other plausible scenario is that both the Gardiners and Wantagh are Sangamon and the Merrick between them is not outwash at all, but a sheet of laterally continuous sand deposited by migrating barrier islands as sea-level rose and fell during the Sangamon.